1. Field of the Invention
An aspect of the present invention relates to at least one of a projection optical system and an image projection apparatus.
2. Description of the Related Art
An image projection apparatus has been known that is capable of being placed nearer a screen than conventional ones. Such an image projection apparatus is called a close-in projector. A close-in projector is such that it is possible to avoid glare of projection light incident on an eye of a presenter (such as a reporter or a speaker) standing near a screen that is a surface subjected to projection on which an image is projected. Furthermore, it is possible for a close-in projector to project an enlarged image without placing the projector near an audience listening to the presentation of a presenter, and hence, it is possible to be used in such a manner that exhaust or noise from such an image projection apparatus does not influence the audience.
A projection optical system included in a close-in projector is such that an angle of view of a conventional (coaxial or rotationally symmetric) projection optical system is increased to reduce its distance from a screen, a curved mirror is used to be capable of projecting an enlarged image even at a close range, or the like. In a case where an angle of view of a conventional projection optical system is increased, an outer diameter of a lens at a side near a screen may have to be increased, and hence, a total size of such a projector may be increased. On the other hand, as long as a curved mirror is used, it may be possible to have a compact projector and conduct projection in a close range.
For example, projection optical systems described in Japanese Patent No. 4329863, Japanese Patent Application Publication No. 2007-079524, Japanese Patent Application Publication No. 2009-251457, and Japanese Patent Application Publication No. 2010-085973 have been known that use a curved mirror. A projection optical system with a refractive optical system combined with a concave mirror is described in any of Japanese Patent No. 4329863, Japanese Patent Application Publication No. 2007-079524, Japanese Patent Application Publication No. 2009-251457, and Japanese Patent Application Publication No. 2010-085973.
Projection optical systems in Japanese Patent Application Publication No. 2009-251457 and Japanese Patent Application Publication No. 2010-085973 are projection optical systems that are telecentric with respect to an image display element. Projection optical systems in Japanese Patent Application Publication No. 2009-251457 and Japanese Patent Application Publication No. 2010-085973 are such that an aperture stop is arranged near a center of a refractive optical system in a direction of an optical axis thereof. In such a telecentric optical system, all of principal rays that are directed from an image display element to a projection optical system are perpendicular to the image display element. Therefore, among lenses that compose a telecentric projection optical system, a diameter (lens diameter) of a lens near an image display element tends to increase. However, a telecentric projection optical system may be such that light beams are entirely condensed at a position away from an image display element, and hence, it is possible to decrease a lens diameter of a lens arranged near a concave mirror.
On the other hand, projection optical systems in Japanese Patent No. 4329863 and Japanese Patent Application Publication No. 2007-079524 are projection optical systems that are non-telecentric with respect to an image display element. Projection optical systems in Japanese Patent No. 4329863 and Japanese Patent Application Publication No. 2007-079524 are such that an aperture stop is arranged at a position nearer a side of an image display element than around a center of a refractive optical system in a direction of an optical axis thereof. Such a non-telecentric projection optical system may be such that light beams that are directed from an image display element to the projection optical system are entirely condensed at a position near the image display element. Therefore, light beams that entirely leave from an image display element spread naturally, and hence, a diameter of a lens at a position near a concave mirror may be increased. Thus, in a non-telecentric projection optical system, there may be a larger difference between a lens diameter of a lens arranged at a position closest to an image display element and a lens diameter of a lens arranged at a position closest to a concave mirror.
A value of a “projection distance” to a screen that is a surface subjected to projection divided by a “transverse width” of an image projected on the screen is referred to as a “throw ratio”. A projection optical system in Japanese Patent No. 4329863 is such that a throw ratio thereof may be smaller as being “0.5”, and a difference between a “necessary degree of convergence of a light beam” for a light beam directed to a top end of a projected image and a “necessary degree of convergence of a light beam” for a light beam directed to a bottom end of the projected image may be smaller. Therefore, it may be possible to correct a distortion aberration or a curvature of field comparatively simply. However, in a case of a projection optical system with a throw ratio less than “0.35”, it may be necessary for a refractive optical system and a concave mirror to separate a light beam directed to a top end of a projected image from a light beam directed to a bottom end of the projected image certainly and conduct a different control (of a degree of conversion or a direction of a light ray). Accordingly, a lens diameter of a lens close to a concave mirror and a size of a concave mirror may further be increased.